This invention relates generally to the control of electromagnetic radiation and more particularly to enclosures to provide shielding of electromagnetic interference (EMI).
Electronic components generate both electromagnetic radiation and thermal energy (heat) when operated. The electromagnetic radiation can hinder electromagnetic compatibility (EMC) of a system by creating EMI. EMI hampers the operation of other electronic components which are mounted on the same printed circuit board (PCB) or are mounted on other PCBs within the same electronic enclosure. Similarly, the heat generated by the operation of a component can hamper the operation of the component itself or other components.
The most common manner of cooling components is to use convection, that is, to cool the components by blowing air across the surface of the components. Where a PCB is installed in an electronic enclosure, the airflow is provided by fans mounted perpendicular to the PCB.
Various ways have been developed to shield other components from the EMI produced by a component.
One method of providing EMI shielding is to encase a component within a sheet metal box. The component is first soldered in place on the PCB. A sheet metal box, which is slightly larger than the component and has a top and four sides, is placed over the component and soldered to the PCB. There are difficulties with this solution. First, if the component is a high power component, i.e. it generates significant thermal energy, the sheet metal box prevents the component from being directly cooled by convection. The convective air impacts only on the sheet metal box and not on the component itself. Second, if the component is thought to be defective during testing or in operation, it is necessary to unsolder the sheet metal box in order to trouble shoot the component.
Another method of providing EMI shielding is to create an electronic module by encasing the front, top, bottom and sides of a PCB in a sheet metal box. To enable convective cooling of the components, the top and bottom sheet metal pieces have holes punched or drilled through them. However, there are limitations to such a structure. First, the amount of open area which can be generated by this method does not enable sufficient airflow to cool high power components. Second, sheet metal with holes does not provide sufficient EMI shielding.
Where an electronic system which is generating electromagnetic energy is large, it is known to provide EMI shielding by placing the electronic system in a box the sides of which consist of a metal formed to have honeycomb openings through it (honeycomb material). The metal provides EMI shielding while the honeycomb openings enable airflow.
In such applications, the sides of the box are held together in a sheet metal frame. To create the sheet metal frame, a piece of sheet metal is bent over the edges of the box thereby covering a portion of the honeycomb openings. The overlap of the sheet metal frame over the sides of the box must be sufficiently large in size to support the box.
This structure is acceptable for large enclosures because the percentage of honeycomb openings which are covered by the sheet metal frame is insignificant. However, where the enclosure is small, an unacceptably large percentage of the honeycomb openings are covered by the sheet metal frame preventing the desired airflow.
The present invention provides an electronic module which has increased capacity for convective cooling and increased EMI shielding relative to previous options. To achieve these advantages, the present invention uses a honeycomb material held within a solid metal frame for the top and the bottom of an electronic module. The term solid metal frame is used to refer to a frame which is machined from a block of metal or is cast metal. The resulting frame is stronger and can be made thicker while still being manufactured to tighter tolerances than a frame which is formed from bending sheet metal. It can therefore be designed to minimize the lateral area covered by the frame and to hold the honeycomb material with a very small lip or no lip. This maximizes the size of the openings and minimizes the overlap of the frame on the honeycomb material thereby maximizing the exposed honeycomb openings. The result is that the honeycomb material held in a solid metal frame may be used to provide combined EMI shielding and airflow in smaller areas than honeycomb material could previously be used, in particular in the top and the bottom of a module.
Advantageously, the small hole size and greater thickness of the honeycomb material offers higher EMI shielding than a drilled or punched metal sheet. Another advantage of the structure of the present invention is that the honeycomb material has more open spaces for airflow than a drilled or punched sheet metal.
The invention may be summarized according to a first broad aspect as a module for enclosing electronic systems which generate electromagnetic radiation and thermal energy comprising: an electrically conductive metal front, an electrically conductive metal left side mechanically and electrically connected to said front, an electrically conductive metal right side mechanically and electrically connected to said front, a rear opening, a top frame of electrically conductive solid metal which is mechanically and electrically connected to said front, said left side and said right side, said top frame having at least one opening, electrically conductive honeycomb material received in and substantially filling the at least one opening in said top frame and being mechanically and electrically connected to said top frame, a bottom frame of electrically conductive solid metal which is mechanically and electrically connected to said front, said left side and said right side, said bottom frame having at least one opening, and electrically conductive honeycomb material received in and substantially filling the at least one opening in said bottom frame and being mechanically and electrically connected to said bottom frame, whereby the honeycomb material provides electromagnetic radiation shielding while permitting airflow therethrough.
The invention may be summarized according to a second broad aspect as a frame and honeycomb assembly for use with a module comprising: a frame of electrically conductive solid metal, and electrically conductive honeycomb material received in and substantially filling an at least one opening in said frame and being mechanically and electrically connected to the said frame, whereby the honeycomb material provides electromagnetic radiation shielding while permitting airflow therethrough.